The combat uses for helicopter aircraft have changed dramatically over the years to include contact with opposing forces, including reconnaissance and combat aircraft assistance of troops on the front line. This type of use subjects helicopters to numerous threats, and therefore new helicopter designs incorporate offensive weapons, such as Gatling guns and rocket launchers.
Initially, the primary control of helicopter weapons was accomplished by the pilot aiming the aircraft at the target prior to firing. Correction for misses was accomplished by the pilot adjusting the attitude of the aircraft prior to expending additional ordnance. As technology developed, tracking and sensing systems were used to locate the target and determine the aircraft attitude necessary to aim the weapon so as to account for outside forces acting on the ordnance, e.g., wind, aircraft speed, etc. Such a system typically displays a "cross-hair" indicative of actual aircraft attitude and a geometric shape indicative of the required aircraft attitude to provide a high probability of striking the target with the weapon. The pilot is required to maneuver the aircraft so as to place the cross-hair in the firing solution defined by the shape prior to firing the weapon. The aiming instructions e.g., cross-hair and geometric shape, are typically displayed on a control panel, a heads-up display, or helmet-mounted display which provides the pilot with visual information relating to the target position, ownship attitude, heading, speed and altitude.
Although such aiming systems improved weapons delivery accuracy, the pilot is still under a significant burden to regulate aircraft heading and pitch attitude. It is well-known that a skilled helicopter pilot can control aircraft attitude within about 1 degree of pitch and yaw. Although this may seem very accurate control, a 1 degree variation in pitch or yaw will have a significant effect on the trajectory of a projectile.
When the pilot is maneuvering the aircraft for targeting, the aircraft automatic flight control system (AFCS) will typically provide control signals such that the aircraft executes coordinated turns. A coordinated turn for a rotary wing aircraft, i.e., a helicopter having a single main rotor, is defined as a banked turn where the body of the aircraft is tangential to a curvilinear flight path of the aircraft, i.e., no side-slip vectors. Control of yaw axis commands to the tail rotor is critical in this type of maneuver.
In mechanical linkage control systems a coordinated turn requires that the pilot simultaneously input, via rudder pedals, the proper amount of yaw to match the amount of roll input provided via the cyclic stick. The more recent fly-by-wire flight control systems (see e.g., U.S. Pat. Nos. 4,203,532, 4,067,517, 4,206,891 and 4,484,283, all assigned to the assignee of the present invention) automatically provide the matching yaw input. The AFCS commands a coordinated yaw input, typically at air speeds above 60 knots, based on the sensed rate of the yaw rate gyro. The coordinated yaw signal is then used to modify the main and tail rotor command signals as necessary to drive the helicopter's lateral acceleration to zero.